Conventionally, fiber optic links use fibers that rely on total internal reflection for confinement of an optical propagation mode. Therefore, the structure of the fibers uses a high refractive index core of approximately five to one hundred microns in diameter, surrounded by a low refractive index cladding. The cladding is present mostly to provide mechanical stability to the fibers. Thus, in a conventional fiber, if the cladding is removed, light is still confined to the high index core suffering very little loss since the surrounding air also has a very low refractive index. Disadvantageously, a conventional fiber optic link can be tapped into quite easily. For example, the fiber may be exposed from its cable, removing an outer polymer protective coating. The glass cladding layer may be thinned via mechanical or chemical methods. A second strand of fiber with removed cladding may be brought in close proximity to the fiber being tapped, and a small fraction of light will be coupled out of the tapped fiber into the second strand. While the light coupled out reduces transmitted light intensity in the tapped fiber, the coupling loss may be extremely small such as on the order of one percent or less. Here, an intruder may monitor the coupled out light. Conversely, the intruder may inject parasitic light into the tapped fiber to disrupt or spoof existing optical carriers. This small one percent or less loss induced in the transmitted light is easily obscured by natural optical power variations associated with thermal and mechanical stresses, optical amplifier noise, etc. Therefore, network operators and end users may be completely unaware of a compromised link.
Conventional fiber optic security includes various systems and methods. First, optical signal light intensity may be monitored for decreases in power (e.g., one percent or less). Unfortunately, this method has limited resolution capability as intrusion events may be easily obscured by natural system mechanical and thermal perturbations, optical amplifier noise, etc. Thus, low alarm threshold settings will produce many false positives whereas high alarm threshold settings will miss intrusion events. Second, an external probe light may be injected into a fiber optic link. This probe may be conditioned to be less sensitive to natural system perturbations. Disadvantageously, the probe solution requires a large amount of dedicated hardware with a separate deployment required for each segment of a large network. Third, quantum key distribution may be used as a secure communication method. Such a system is very complex and provides only extremely slow kilobit rate communications over short distances (tens of kilometers). Thus, it is only suitable for distributing encoding information (i.e., keys) and not for carrying actual data. Finally, fiber-based strain sensors, such as ones based on Brillouin optical time domain analysis, may be used for intrusion detection. Glass material strain induces a frequency shift in light backscattered by the Brillouin effect allowing for identification of induced fiber strain location. However, this method only indicates that a fiber is being mechanically handled and may provide false alarms in cases where fiber is not mechanically isolated (e.g., aerial cables, subway ducts, etc.).